RF describes an antibody (IgM, IgG and IgA) that is directed toward antigenic determinants present on human and animal IgG. RF-IgM activity is detected by latex agglutination, nephelometry, radioimmunoassay (RIA) or enzyme linked immunosorbent assay (ELISA), of which ELISA and RIA have been proposed as the best techniques to measure RF-IgM activity. The diagnostic specificity (20%) and predictive value (10%) of an RF-IgM test for RA is low. The specificity and predictive value of the RF test may be increased, however, by simultaneously measuring the three RF isotypes: RF-IgM, RF-IgA and RF-IgG. When all three RF isotypes are elevated, the specificity and predictive value of RF testing improves substantially. See Cyclic Citrullinated Peptide Antibodies, Serum
Separate serum from cells ASAP or within 2 hours of collection.